High tension low temperature tough steel



United States Patent 3,348,981 HIGH TENSION LBW TEMPERATURE TOUGH STEEL Susumu Goda, Hisashi Gondo, and Yasumitsu ()noue, Kitakyushu, Japan, assignors to Yawata Iron & Steel Co., Ltd., Tokyo, Japan Filed Feb. 17, 1965, Ser. No. 433,303 Claims priority, application Japan, Feb. 21, 1964, 39/ 9,642 2 Claims. (Cl. 148-36) ABSTRACT OF THE DISCLOSURE A high tensile strength, low temperature tough steel consisting essentially of 0.05 to 0.25% by weight of C., 0.04 to 0.17% by weight of Al, 0.05 to 1.60% by weight of Mn, 0.20 to 1.60% by weight of Cr, and 0.05 to 1.00% by weight of M0, the combined weights of Mn, Cr and Mo being no greater than 2.5% by weight, the remainder being Fe and inevitable impurities, and the steel having the aluminum in solid solution therein, the steel having been heated to a temperature above the A point to austenitize it, thereafter hardened at a cooling rate of -60 C. per sec., and then tempered by heating to a temperature between 500 C. and the A point.

This invention relates to a high tension low temperature tough steel.

In general, a steel material high in toughness at low temperature is needed for making tanks for use in storing liquefied petroleum gas of low boiling point. Heretofore, for this purpose a steel containing nickel or other alloying elements in a large amount or low carbon steel low in strength has been used. However, the use of steels of these kinds involved such economic disadvantages that not only the production cost was high, but also a thick plate was required.

An object of the present invention is to provide a steel having sufiicient strength, toughness and weldability at a low production cost by reducing the addition of expensive alloying elements.

Other objects of the present invention will be made clear by the following explanation with reference to the attached drawings.

FIG. 1 is a diagram showing the relationship between the content of aluminum and fracture transition temperature and FIG. 2 a diagram showing the relationship between the cooling rate during hardening and the fracture transition temperature.

The steel according to the present invention is a high tension low temperature tough steel obtained from a steel material, which consists essentially of 0.05 to 0.25 wt. percent C., 0.10 to 0.70 wt. percent Si, 0.04 to 0.17 wt. percent Al, 0.05 to 1.60 wt. percent Mn, 0.20 to 1.60 Wt. percent Cr, 0.05 to 1.0 wt. percent Mo with the Mn+Cr+Mo being 2 2.5 wt. percent, as the basic constituents and further consisting of 0.03 to 0.15 wt. percent V, if necessary, and the rest being Fe and inevitable impurities, by heating said steel material to above the A, point to austenitize the same, thereupon subjecting the thus austenitized steel to a hardening at a cooling rate of from 10 to 60 C./sec. and then tempering the hardened steel in a temperature range of 500 C. to the A, point.

Among the above enumerated constituents, the range for C of 0.05 to 0.25 wt. percent is determined from the following necessities, that is, more than 0.05 wt. percent C is needed to maintain the hardenability, but the upper limit is determined to be 0.25 wt. percent from the view point of the weldability. More than 0.01 wt. percent Si is "ice needed from the point of view of steel making, but more than 1.5 wt. percent is not desirable from the point of view of the toughness.

Mn is an economic element for use in effectively increasing the tensile strength. The addition of more than 0.5 wt. percent Mn displays the effect, but if the addition exceeds 1.5 wt. percent, the weldability will be impaired.

Cr is effective in increasing the strength of steel, if the addition exceeds 0.20 wt. percent, but more than 1.60 wt. percent is not desirable from the point of view of the notch-toughness. 0.05 to 1.0 wt. percent M0 is effective not only in increasing the strength but also improving the toughness.

Particularly, as regards Mn, Cr and M0 the sum of Mn+Cr+Mo is limited to 2.5 wt. percent in order to maintain the strength, low temperature toughness and weldability and to prevent the formation of excessive martensite. As is well known, when making a molten alloyed steel it is usual that a small amount of Ni such as 0.01 to 0.04 wt. percent is contained in the steel. Therefore, in such a case it is desirable to limit the sum of Mn+Cr+Ni to less than 2.5 wt. percent.

V is added to increase the strength, but the impact value will be somewhat deteriorated thereby. Consequently, V is to be added in an adequate amount according to the intended use of the steel produced. A range of from 0.03 wt. percent to 0.15 wt. percent is a suitable content of V for producing a steel having a relatively good impact value.

The addition of Al is one of the most important features of the present invention. The steel of the present invention contains Al more than any normal fine grained steel, that is, in a range of from 0.04 to 0.17 wt. percent. Al should be in a state of solid solution in the steel. Therefore, the content of N or 0 which eliminates the effect of Al by combining with Al, should be made as small as possible, and if these are coexistent, Al should be added in excess. The content of Al in an amount less than 0.04 wt. percent produces no effect, but if the content of Al exceeds 0.17 wt. percent, the viscosity of molten steel is increased when producing steel, which is apt to produce material defects or non-metallic impurities. Therefore, it is desirable to specify the upper limit of A1 as 0.17 wt. percent.

The cooling rate for hardening the material heated to be austenite is specified as being in a range of from 10 to 60 C. per second, preferably from 15 to 25 C. per second. Then, the material quenched from austenite is tempered in a temperature range from 500 C. to the A transformation point, whereby the desired strength and toughness may be simultaneously obtained.

FIG. 1 shows the effect of the Al content on the fracture transition tempera-ture according to the V-notch Charpy test made on the steels which have a composition in the range specified by the present invention among those shown in Table 1. As evidently seen from the said FIG. 1, the fracture transition temperature is below C. when the content of Al is in the range of from 0.04 to 0.15 wt. percent. No effect can be demonstrated even if the content of Al exceeded 0.17 wt. percent.

The steel 'D in Table 2 is that which has been obtained by especially adding Mn supplementally to a steel of the same composition as the steel D and in which the sum of Mn-l-Cr-i-Mo amounts to 2.7%. In this steel a deterioration was perceived in the low temperature toughness, even if the cooling rate was 45 C. per second.

FIG. 2 shows the fracture transition temperatures in correlation with the cooling rate in hardening the steels D and I shown in Table 1. The steel I which contains V showed a deterioration in the impact value as compared with the steel D which does not contain V. However, the former steel showed a remarkable improvement in. the tensile strength, as shown in the said figure. Therefore, the addition of V may be considered according to the intended 4 C, 0.10 to 0.70% by weight Si, 0.04 to 0.17% by weight of Al, 0.03 to 0.15% by Weight of V, 0.05 to 1.60% by weight of Mn, 0.20 to 1.60% by weight of Cr, and 0.05 to 1.00% by weight of M0, the combined weights of Mn,

use of the steel. 5 Cr and Mo being no greater than 2.5% by weight, the

TABLE 1 [Relationship between the amount added of Al and mechanical properties} Chemical composition (wt. percent) Heat-treatment Tension test V-notch Charpy test Sample Water Temper- Cooling Yield Tensile. Elonga- Fracture C Si Mn P S Cr Mo Al V hardening rate point strength tion transition g Temp. C.) (kg./ (kg/mm!) (pertemp. C.) C.) mm!) cent) C.)

TABLE 2 [Relationships between the cooling rate hardening, the amounts added of alloying elements and mechanical properties] V-notch Hardening Cooling Tempering Yield Tensile Charpy test, Sample temperature rate temperature point strength Elongation Fracture Mn+Cr+Mo 0.) C./sec.) C.) (kg./mm. (kg/mmfl) (percent) transition (percent) temperature What is claimed is:

1. A high tensile strength, low temperature tough steel consisting essentially of 0.05 to 0.25% by weight of C, 0.04 to 0.17% by weight of Al, 0.05 to 1.60% by weight of Mn, 0.20 to 1.60% by Weight of Cr, and 0.05 to 1.00% by weight of M0, the. combined weights of Mn, Cr and Mo being no greater than 2.5 by weight, the remainder being. Fe and inevitable impurities and thesteel having the aluminum in solid solution therein, the steel having been heated to a temperature above the A point to austenitize it, thereafter hardened at a cooling rate of 10-60" C. per sec., and then tempered by heating to a temperature between 500 C. and the A point.

2. A high tensile strength, low temperature tough steel consisting essentially of 0.05 to 0.25% by weight of References Cited UNITED STATES PATENTS 2,770,563 11/1956 Herzog l24 X 3,110,635 11/1963 Gulya 14836 3,251,682 -5/1966 Wada 75l24 3,258,372 6/1966 Miller et al. 148-36 CHARLES N. LOVE-LL, Primary Examiner. 

1. A HIGH TENSILE STRENGHT, LOW TEMPERATURE TOUGH STEEL CONSISTING ESSENTIALLY OF 0.05 TO 0.25% BY WEIGHT OF C, 0.04 TO 0.17% BY WEIGHT OF AL, 0.05 TO 1.60% BY WEIGHT OF MN, 0.20 TO 1.60% BY WEIGHT OF CR, AND 0.05 TO 1.00% BY WEIGHT OF MO, THE COMBINED WEIGHTS OF MN, CR AND MO BEING NO GREATER THAN 2.5% BY WEIGHT, THE REMAINDER BEING FE AND INEVITABLE IMPURITIES AND THE STEEL HAVING THE ALUMINUM IN SOLID SOLUTION THEREIN, THE STEEL HAVING BEEN HEATED TO A TEMPERATURE ABOVE THE A3 POINT TO AUSTENITIZE IT, THEREAFTER HARDENED AT A COOLING RATE OF 10-60* C. PER SEC., AND THEN TEMPERED BY HEATING TO A TEMPERTURE BETWEEN 500*C. AND THE A1 POINT. 